The subject matter of the present invention is binding agents on the basis of chelated titanic acid esters. These binding agents are used both for the manufacture of ceramic moldings, such as for example casting molds or casting cores for metal casting, and in internal coating compositions for the protection of, for example, crucibles made of material that is not resistant to molten metals, in which aggressive metals or alloys are to be melted.
It is known to cast metals in ceramic molds. For the manufacture of such molds, finely divided refractory material is mixed with a usually silicatic binding agent, and the mixture is then applied to a model by immersion or spraying or by pouring it over the model. Such molds are hardened by simply drying them, or by applying a gaseous, liquid or solid hardener, such as ammonia, such that a gel is formed of the liquid binding agent, which, in the case of the silicatic binding agents commonly used, is converted in the firing to silicon dioxide. In the lost-wax or investment molding process that is frequently used, the wax model is built up with layer after layer of ceramic slip by repeated dipping, each layer being sprinkled, while still moist, with coarse refractory grits and dried. After the last layer is dry, the model is melted out, and the mold is fired in a kiln.
However, satisfactory casting results are not always obtained with the molds and cores made by these known methods. Especially in the casting of high-alloy steels and steels of high carbon or chromium content, and in the casting of titanium, reactions with components of the mold frequently occur, so that the castings thus obtained have flawed surfaces.
These flaws can also occur when the said metals or alloys are first melted down or held in the molten state in crucibles made of siliceous material, such as clay for example, before they are poured.
One known possibility of preventing such defects is the use of silicon-free binding agents for the molds or coatings. The derivatives of titanium, aluminum or zirconium which are similar to ethyl silicate, however, are far more sensitive to hydrolysis than the organic silicates, since they are degraded merely by atmospheric moisture. Therefore it is not easily possible to adapt the methodologies known in silicate chemistry to these binding agent starting materials. The method described in German Patent No. 2,204,531 indicates an approach to overcoming this difficulty by reducing the sensitivity to hydrolysis of aluminum sec.-butylate, zirconium n-butylate or zirconium n-propylate and tetra-isopropyl titanate, for example, by adding to them certain chelating agents, to such an extent that they can be used in preparing binding agents, or they can be mixed with appropriate refractory materials for the preparation of silicon-free slips. The slips made with these silicon-free binding agents, however, have the disadvantage that, when they are used for the internal coating of clay or mullite crucibles, they have, after firing at 700.degree. C., surfaces which are either relatively soft or crazed and thus unable to serve their purpose in an optimum manner.
The problem therefore was to find a binding agent for slips of refractory material, which, after the slip made from it has hardened, yields casting molds for aggressive metals or metal alloys, such that these molds will have a surface that is strong, free of crazing, and resistant to attack by the said metals in the molten state. It is also to be possible to use the slips for preparing internal coatings in crucibles or other vessels made of clay or siliceous material and serving to contain aggressive molten metals or metal alloys.